Data sent through a channel is subject to linear distortion as well as other impairments. An “equalizer” is a device which is used to remove, or reduce (hereafter collectively referred to as “compensate”) linear distortion and is commonly made a part of a receiver.
One type of equalizer is called an “adaptive” equalizer. Adaptive equalizers comprise “tap coefficients” that are continuously adjusted, with the goal of producing an error free (i.e., distortion-free) output signal.
Adaptive equalizers typically operate in various modes, two of which are a “decision directed” and “blind” mode.
Decision directed, adaptive equalizers make use of an “ideal” reference signal. For example, a pseudo-random sequence may be used as an ideal reference. Sometimes, however, it is not practical to use an ideal reference. Blind, adaptive equalizers may then be used. Such equalizers use a “fixed” reference. The value of the fixed reference always remains the same as compared with an ideal reference, whose value may change randomly. Both decision directed and blind adaptive equalizers require a “start-up time”. It has been known for some time that adaptive equalizers require a certain start-up time before they can be used to accurately receive and recreate data. This time is called a “convergence time”. Typically, decision directed, adaptive equalizers have shorter convergence times than blind adaptive equalizers. This is due to the fact that a decision directed, adaptive equalizer's tap coefficients may be adjusted in large increments without creating a large deviation from a mean (i.e., statistical mean) error. In contrast, the tap coefficients of a blind, adaptive equalizer must be incremented in very small amounts resulting in equalizers which take 100 times as long (i.e., two orders of magnitude longer) to reach convergence than decision directed, adaptive equalizers. Efforts to date to increment tap coefficients of a blind, adaptive equalizer in relatively large increments have not proven successful. Existing techniques inherently generate too much noise, which prevents an equalizer from reaching convergence.
It is believed that the convergence time of existing blind, adaptive equalizers takes longer than necessary.
It is therefore a desire of the present invention to provide for methods and devices which shorten the time it takes for blind, adaptive equalizers to reach convergence.